Brightness enhancing films (BEF) have been used in lighting panels for directing light from lighting fixtures through luminaires, laptop computers displays, and flat panel desk top monitors or displays. The brightness enhancing films, which can have linear prisms, diffuse light with a desired directionality. Often the films have been used in combination with a fluorescent light source. The films have had partial success in improving luminaire or display brightness by controlling the angle at which light emerges.
Optical films are often used in combination with other films or sheets. For example, two diffuse films and two light-redirecting or collimating films are used as a stack on a light guide behind liquid crystal displays. When the optical films are positioned adjacent another layer, such as a diffuser or another optical film, defects in the lighting panel are often noticeable by the viewer. One such defect is caused by the optical film “wetting-out”, which can occur when the film contacts an adjacent film. At the point of contact, the change in refractive index is eliminated or substantially attenuated for light passing through the two films. Since the two films are not uniformly contacting one another, defects are noticeable by the viewer.
Another defect that may be present is referred to as “Newton's rings,” which occurs when two films are gradually separated from each other, for example, by a particle such as dust.